1. Field of the Invention
This invention relates in general to a mass storage systems for computer systems, and more particularly to a method and apparatus for providing feedforward control of two interacting actuators to compensate for interaction there between.
2. Description of Related Art
Fixed magnetic disk systems, typically referred to as xe2x80x9chardxe2x80x9d disk drives, are now commonplace as the main non-volatile storage in modern personal computers, workstations, and portable computers. Such hard disk drives are now capable of storing gigabyte quantities of digital data, even when implemented in portable computers of the so-called xe2x80x9cnotebookxe2x80x9d class. Many important advances have been made in recent years that have enabled higher data density and thus larger storage capacities of hard disk drives, and that have also enabled much faster access speeds, both in the bandwidth of data communicated to and from the hard disk drive, and also in the access time of specified disk sectors. Advances have also been made that have greatly reduced the size and weight of hard disk drives, particularly as applied to portable computers, have been made over recent years. These advances have resulted in the widespread availability of ultra-light portable computers, yet having state-of-the art capability and performance.
A head/disk assembly typically comprises one or more commonly driven magnetic disks rotatable about a common spindle and cooperating with at least one head actuator for moving a plurality of heads radially relative to the disks so as to provide for the reading and/or writing of data on selected circular tracks provided on the disks. As track pitch decreases, servo performance will decrease because of the more stringent tracking criteria unless a break is made with current disk drive design. One proposal has been to put two independently controlled actuators on one disk drive. Servo performance is then increased because one actuator can seek to a new location while the second actuator is accessing data. In this way the seek and settle time is hidden from the user: he sees instead only the final portion of the seek that occurs when the data access on the other actuator has completed.
In some disk drives, a plurality of head actuators are employed. For example, in one type of disk drive, head actuators are located at different circumferential positions relative to the disks. Other types of disk drives provide a combination of rotary and linear actuators. Still other disk drives use a plurality of head actuators, wherein at least a first actuator position a first set of transducers relative to a first group of rotating disks and at least a second actuator position a second set of transducers relative to a second group of rotating disks. However, in all cases, to obtain greater system flexibility, it is advantageous to provide for each head actuator to be separately controllable so that its respective heads can be moved to a desired track irrespective of the track position provided by any other head actuator for its respective heads.
A primary function of a disc drive actuator and its accompanying servo control system is to keep the read/write heads on track. However, the primary difficulty with multiple, independent head actuators is that the act of seeking by one actuator adversely affects the positioning of the second actuator. If severe enough, this interaction can cause data errors on the actuator that is accessing data. Less severe but still undesirable is the loss of performance because of soft errors. Mechanical resonances within the actuator limit the precision of the tracking system and adversely affect the time required for the heads to settle after a seek. Many actuators, and most notably rotary actuators, have relatively long and flexible head arms. Such elongated head arms tend to have separate modes of vibration which are easily excited during normal operation of the disc drive. When such vibrations go undamped or are only lightly damped, they limit the overall performance of the disc drive.
In the past, such control problems were typically overcome by limiting the bandwidth of the control system to movements that are not likely to excite such resonances. However, when relatively elongated head arms are used and as track pitch decreases, merely limiting the performance of the control system will have an adverse effect on the overall drive performance.
For example, a plurality of separately controllable head actuators may be mounted on a common (or mechanically interconnected) base structure. Thus, the driving of one head actuator will cause at least some portion of the mechanical driving forces to be coupled to other head actuators via the common base structure. A head/disk assembly and its associated servo control are typically constructed and arranged so that coupling forces produced as a result of driving a head actuator have a minimal effect on the operation of any other head actuator.
Nevertheless, because of the demand for greater data storage density and faster access speed in a disk storage system, head/disk assemblies are being designed with reduced track-to-track spacings to increase density and larger actuator driving forces to reduce access time. As a result, the driving forces coupled from one head actuator to another are that much more difficult to handle.
It has also been found that when a head actuator is to be driven an appreciable radial distance, the large driving force required to provide rapid access may cause a sufficiently large force to be coupled to another head actuator so as to displace its heads off their current track by an amount which cannot be compensated for by the servo control. Of course, it may be possible to redesign the mechanical structure of the head/disk assembly and/or the servo control circuitry to compensate to a certain degree. Still, the added expense resulting from such designs is considerable, particularly since redesign typically requires extensive testing and field experience to insure reliability. Moreover, the interaction between actuators still exists and, as described above, the trend in decreasing track pitch requires a solution that can more effectively control the accurate operation of multiple, independently controlled head actuators.
It can be seen that there is a need for a servo controller that counteracts the effects of another actuator by filtering the other actuator""s acceleration.
It can also be seen that there is a need for a method and apparatus for providing feedforward control of two interacting actuators to compensate for interaction there between.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for providing feedforward control of two interacting actuators to compensate for interaction there between.
The present invention solves the above-described problems by providing state models for each actuator that processes acceleration for each actuator to provide an estimate of head acceleration for each of the actuators and to provide feedforward control signals to the actuators for controlling the positioning of the heads based upon the estimates of head acceleration for each of the actuators.
A system in accordance with the principles of the present invention includes at least one rotatable disk, at least first and second separately movable heads for providing transducing action with respect to the disk, first and second head actuators mounted so that the operation of one actuator causes mechanical forces to be coupled to the other actuator, said first actuator providing for radial movement of said first head relative to said disk and said second actuator providing for radial movement of said second head relative to said disk, the radial movement provided by each actuator being separately controllable from that of the other actuator and at least one controller, wherein the at least one controller comprises a first and second state model associated with the first and second actuators, wherein the first state model receives an acceleration applied to the second actuator and the second state model receives an acceleration applied to the first actuator, the first and second state models generating head acceleration estimates for the first and second actuators, and wherein the at least one controller produces control signals for the first and second actuators based upon the head acceleration estimates for the first and second actuators.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the first and second state models produce head acceleration estimates for the first actuator by filtering the effect of the second actuator and wherein first and second state models produce head acceleration estimates for the second actuator by filtering the effect of the first actuator.
Another aspect of the present invention is that the at least one controller comprises a single controller, the single controller implementing both the first and second state models.
Another aspect of the present invention is that the at least one controller comprises a first and second controller, wherein the first controller implements the first state model for the first actuator and the second controller implements the second state model for the second actuator.
Another aspect of the present invention is that the system further includes a first and second register, wherein the first controller writes an acceleration for the first actuator in the first register and the second controller writes an acceleration for the second actuator in the second register, the first controller reading the second register and applying the acceleration stored in the second register to the first state model to produce the head acceleration estimates for the first and second actuators.
Another aspect of the present invention is that the second controller reads the first register and applying the acceleration stored in the first register to the second state model to produce the head acceleration estimates for the first and second actuators.
Another aspect of the present invention is that the at least one controller comprises a first and second controller, wherein the first controller implements the first and second state models for the first and second actuators.
Another aspect of the present invention is that the second state model implemented in the first controller receives from the second controller an acceleration applied to the second actuator, the first state model receiving from the first controller an acceleration applied to the first actuator, the first and second state models producing estimates of head acceleration for the first and second actuators, the second state model providing a feedforward control signal to the second controller that is used by the second controller to generating a control signal for the second actuator.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.